Signaling circuit



March 5,, WW0 H. o. PETERSON 51' s.

SIGNALING CIRCUIT Original Filed Oct. 26, 1933 2 Sheets-Sheet 1 as 6/4 4.4 we

J0 70 IND/(470R ascvu/lrae WORK CIRCUIT -lillllnlniL 5 EL 567K002: Pom/mus YNVENTORS MARTIN KATZIN AND M H. o. PETERSON MSH'bRNEY Patented Mar. 5, 1940 UNITED STATES PATENT OFFlCE SIGNALING CHRCUIIT Harold Olaf Peterson and Martin Katzin, Riverhead, N. Y., assignors to Radio Corporation of America, a corporation of Delaware Original application October 26, 1933, Serial No. 695,254. Divided and this application August 28, 1936, Serial No. 98,266

16 Claims. (01. 25il-20) This invention relates to the art of signaling diagram showing the application of practical poand more particularly to the art of modulation tentials, and curves derived by operation of the and demodulation of carrier oscillations by means tube of 2a under certain conditions;

of thermionic tubes, and more specifically to the Figure 3 is a modification of the arrangement methods for modulating and demodulating carof Figure 1; nor frequencies which employ multi-grid tubes. Figure 4 illustrates the manner in which the This application is a division of our United States principles of the present invention is applied to apphcation #695,254, filed October 26, 1933, now demodulation circuits of the non-heterodyne Patent #2103369, issued December 28, 1937. The type; while,

invention makes possible eilici-ent and complete Figure 5 is a curve illustrating the operation of 10 linear demodulation of a signal modulated carthe device of Figure 4. nor wave. In this demodulation spurious re- The present invention may best be described spc-nses known as harmonic points are eliminated. by reference to the drawings of Figures 1, 2a The method of demodulation is particularly apand 2b. plicable to receivers of the heterodyne type. Figure 1 shows one specific embodiment of the 1 Demodulation systems of the heterodyne type invention in which a complete teleph y receiver known. heretofore in some cases do not produce is represented. Referring to Figure l, A is a linear demodulation and such systems may rethermionic tube with which the associated input spend to interfering signals as well as the desired circuit 5 coupled to aerial 6 forms a conventional to signal if the interfering signal is at a frequency signal Wave amplifying circuit whereinwave en- 29 separated from the local frequency byadiiference orgy modulated in H-CCO G JIC With Signals is frequency equal to the beat frequency. Or the intercepted by induced in 5, amplified in A desired signal may appear at two tllfierent setan pp rs i the tuned circuit TC connected tings of the tuning elements. with the anode l of A. The tuned circuit TC 1.1 An object of the present invention is to acincludes an inductance l3 for g t p y cornplish linear and complete demodulation of winding of a transformer havin a Se y a signal modulated carrier frequency by employwinding 53. Coupling coil It serves to transfer ing thermionic tubes having a multiplicity of energy from the amplifier A to the innermost control grids or electrodes, applying the signal grid 29 of the four-grid thermiOn c tube 2 Wh c IJ) modulated carrier frequency oscillations to one grid is biased to a suitable negative potential of the grids and the local oscillations to a pluwith respect to cathode it by means of a potenrality of grids in such relative amounts and tial source or battery M. The grid 18 is byphases as to result in substantially linea (19- passed for radio-frequency by means of condenser modulation by the heterodyne method free of 22 and choke coil 25. This grid forms a Shield c5 interfering signals and image frequencies. between the grid 29 and the input circuits con- In a modification the signal carrying modulated nected therewith and the other tube elements wave may be applied to a plurality of the grids and-circuits. Grid 18 is maintained at suitable in certain relative amounts and phases, while the positive direct-current potential by means of batlocal oscillations may be applied to a single grid. tery 32, grid i9 is shown as being n1aintained .lu In a further modification the wave to be deat suitable negative potential with respect to the modulated is applied in the desired phase relacathode it by means of battery 3%, while grid l9 tion to a plurality of grids of a demodulator that is maintained at a suitable positive potential relreduces the signal without the use of local osciiative to the cathode It by source 3%. Grid i9 lations. may be the usual suppressor grid, E8 the usual 15 The invention has, as required by lax been screen grid, iii an auxiliary grid electrode bepointed out with particularity in the claims aptween shield grid 18 and suppressor grid l9, and pended hereto. 29 the usual control grid of a tube. Voltage vari- The met ad and mode of operation of the same ations of beat frequency, generated by the oscilwill be best understood from the specification lator 0 comprising tube T and its regenerative (3U therefrom when read in connection with the circuits, are applied to the two grids it and i9 5 attached drawings, throughout which like referby coupling between inductance I in the oscillator ence characters indicate like parts, and in which: circuit and inductance I coupled to grids l9 and Figure 1 shows schematically a demodulator i9. By means of this coupling beating voltages including features of the present invention; are applied to both grids l9 and i3 in phase. Figures 2a and 2c are respectively, a wiring Plate 26 of tube ii is connected by a choke in- 55 ductance 34 and radio-frequency by-pass condenser 35 to an intermediate frequency amplifier 50 in which the wave energy of intermediate frequency resulting from the beating action in tube 20 appears for subsequent utilization.

The operation of the demodulator tube 2i in Figure 1 may best be described with reference to Figures 2a and 2b.

The tube T1 of Figure 2a represents the tube 2| of Figure 1 and shows the connection of the various voltages to the several electrodes of the tube including grids 29, i9 and I9 represented in Fig. 2a as grids G1, G2 and G3, respectively. The curves of Figure 2b represent the transconductance betweengrid G1 and plate P for various operating potentials on the several electrodes. Since the output voltage produced across the load in the plate circuit of the tube is proportional to the transconductance between the input electrode and the plate, the curves of Figure 2b also represent the voltage output obtainable under the several conditions.

Curve A of Figure 2b shows the variation of transconductance with EgZ, the voltage between grid G2 and cathode K, showing that considerable distortion would be entailed by applying modulating voltage to grid G2. Similarly curve B, showing transconductance versus Egli, the voltage between grid G3 and cathode K, shows that high modulation percentages would be subject to considerable distortion.

Curve C shows the improvement obtained by varying the potentials of G2 and G3 simultaneously by equal amounts. The result is that the curved portion near the cut-off point is reduced and the cut-off made sharper. The reason for this is that as G3 is made more negative, G2 is simultaneously made less positive, and thus assists in the reduction of transconductance. For composite modulation corresponding to curve C, operation at the direct current electrode potentials corresponding to point a on the curve would be most suitable. As a result, the composite curve C would allow higher modulation percentages without distortion than would curve B.

The operation of this device is not restricted to simultaneous variation of the two grid potentials in equal amounts, as described in the specific embodiment above. The basic principle is to vary simultaneously the potentials of a plurality of electrodes in the proper proportions to obtain linear operation and therefore minimum distortion. One example is shown by curve D of Figure 2b, in which the potential of grid G2 is changed by twice the amount as G3. Operation at the direct current potentials corresponding to point I) of this curve would allow substantially complete modulation to be obtained with very little distortion.

In the modification illustrated in Figure 1, the grids i9 and iii are moved (potentially) by the beating or modulating potentials in the same direction.

The upper end of curves C and D in Figure 2b, which remain curved, but are not utilized as part of the modulation characteristic by virtue of the selection of the direct current potentials of the electrodes to correspond to points a and b, respectively, may be further straightened by connecting a negative resistance device in series with electrode G2, so that as the current drawn by this electrode increases due to a positive excursion of the modulating voltage, the voltage applied between electrode G2 and cathode K will be still further augmentedby the negative resistance device, thus still further increasing the transconductance. This would allow moving points a. and 1) higher upon the curve and thus give greater modulated output from the device.

Although the specific embodiment of the modulation circuit described above is to be used with a four-grid tube, it is understood that the principles of this invention are applicable to a tube of any number of electrodes in which linear modulation is obtained by simultaneous variation of the potentials of a plurality of electrodes at the modulation frequency.

Returning to the operation of Figure 1, tube A and associated circuits form a stage of radio frequency amplification. The anode circuit of tube A may be connected with a tunable circuit TC including an inductance I3 coupled with an inductance l3 connected with the control grid 29 of a tube 2i, which in this case acts as a demodulator tube. 0 is an oscillator including a thermionic tube T having output and input electrodes coupled, as shown, to an oscillation producing circuit including frequency determining means. The oscillation producing circuit includes an inductance I coupled, as shown, to an additional inductance I. The inductance I supplies energy from the heterodyne oscillator O by way of inductance I to the grids l9 and E9 of tube 2!. In this case the grid 58 serves as a shield between the signal circuits and the local heterodyne oscillator O. The demodulated signals appear in the anode circuit including the choke i l and capacity 00 and may be impressed on any utilization circuit by way of an intermediate frequency amplifier 50.

The use of two grids l9 and H) fed with energy from the heterodyning frequency oscillator provides a linear operating characteristic as described above. This results in improved demodulation characteristics in the elimination of spurious responses known as harmonic points. A brief explanation of the origin of these spurious responses will show how the use of the multi-grid modulation principle helps to eliminate them.

Suppose we have a superheterodyne receiver whose intermediate frequency amplifier operates at a frequency of kilocycles. Let us tune this receiver to receive a signal S whose frequency is 10,000 kilocycles. The heterodyne oscillator will therefore be set at a frequency B of either 9900 kilocycles or 10,100; let us choose 10,100 kilocycles. If, in addition to the desired signal S, there is also present a signal (undesired) whose frequency I is 10,200 kilocycles, this second signal will also beat with the heterodyne oscillator frequency to form a frequency of 100 kilocycles. This undesired signal I is called the image frequency and it is primarily for the purpose of eliminating this image that radio frequency amplification (and selectivity) is added ahead of the heterodyne demodulator.

Now suppose, that, in addition to the desired signal S, the input to the receiver contains a frequency H of 10,050 kilocycles, separated from the heterodyning frequency 13 by half the intermediate frequency. If the signal grid characteristic is not perfectly linear, that is, if the signal grid to plate transconductance versus signal grid voltage is nct a straight line, and similarly if the heterodyne frequencygrid characteristic is not linear, then there will appear a number of frequencies in the plate circuit among which is one, 2 (II--13) which equals 100 kilocycles. Therefore, the frequency H (10,050 kilocycles) will give rise to interference. Similarly, other interferences will be encountered for frequencies separated from the heterodyning frequency by integral fractions of the intermediate frequency, i. e., A A, A

etc, of the intermediate frequency. These interferences or spurious responses are known as harmonic point responses.

Now if either the signal grid characteristic or the heterodyning grid characteristic is absolutely linear, then these harmonic point responses will not appear. It is to this end that the principle of double grid modulation would prove useful in a superheterodyne receiver. Obviously, either characteristic could be made very nearly linear by using the two grid principle, so that this case would be one where the connections for the signal frequency and heterodyning frequency could be interchanged. Figure 3 shows an arrangement in which the beating oscillations are applied to the grid 29 and the signal carrying oscillations are applied in the desired phase relation on the grids i9 and I9, while l8 serves for shielding purposes.

Also it is evident that the two grid idea could be applied to each frequency so that we would have a five grid demodulator, the middle grid acting as a shield between the two circuits. We have not shown a diagram of this connection, but the extension is obvious.

While the invention has been illustrated as being applicable to heterodyne demodulators in which a four-grid tube is preferably used so that a screening grid may be interposed between the signal modulation carrier receiving circuits and the heterodyne circuits, it will be understood that the invention is also applicable to nonheterodyne demodulation. In such a case a three-grid tube may be used. A signal modulated carrier may be applied to two grids while the inner grid is suitably biased. The biases on the first grids are such that the tube is in the cut-off position when no signal is applied. A circuit of the non-heterodynetype, as, for example, a second detector in a heterodyne circuit,

has been shown for purposes of illustration in; Figure 4.

While we have shown a three-grid tube in this modification, Figure 4, it is understood that grid 29 and its associated biasing battery may be omitted, thus allowing the principles of this modification to be effected with a two-grid tube.

In Figure 4 the radio frequency amplifier A is coupled by way of its output circuit TC to an inductance it which impresses the signal modulated carrier frequency oscillations amplified by A in the desired phase relationship upon the two outer grids l9 and E9. The inner grid 29 is in this case biased to a suitable potential by a source as shown. The direct current potentials for the grids l9 and I9 respectively are fixed by the direct current sources 39 and 39 connected as shown.

The potential of the sources is so selected that the tube is operated in the cut-off position when no signal is applied. This point is indicated at a in Figure 5 of the drawings.

The linear demodulated oscillations may be fed from the anode circuit including the transformer T2 to a work circuit directly or by way of additional amplifiers including one or more thermionic tubes 50 connected with the secondary winding of transformer T2.

We claim:

1. The method of effecting distortionless demodulation of signal modulated carrier frequency oscillations by means of an electron discharge tube having a control grid electrode, a cathode electrode and a plurality of auxiliary grid-like electrodes which includes the steps of, impressing signal modulated carrier frequency oscillations on said control grid electrode, applying other oscillations substantially in phase to a pair of said auxiliary electrodes, and maintaining the same at different direct current potentials with respect to said cathode, one of said auxiliary grid-like electrodes being maintained at a potential equal to the potential of the other auxiliary electrode plus a constant.

2. The method of signaling by means of an electron discharge tube having a cathode electrode and a plurality of grid-like electrodes which includes the steps of, impressing oscillations on one of said grid-like electrodes, applying signal modulated oscillationsof different frequency substantially in phase to a pair of said grid-like electrodes and maintaining the same at different direct current potentials with respect to said cathode, one of said pair of gridlike electrodes being maintained at a potential equal to the potential of the other pair of gridlike electrode plus a constant.

3. The method of effecting distortionless demodulation of signal modulated carrier frequency oscillations by means of an electron discharge tube having a cathode electrode, and a plurality of grid-like electrodes which includes the steps of, impressing the signal modulated carrier frequency oscillations on one of said grid-like electrodes, applying heterodyning potentials of different amplitude substantially in phase to a pair of said grid-like electrodes, and maintaining the same at different'direct current potentials with respect to said cathode, one of said pair of gridlike electrodes being maintained at a potential equal to the potential of the other of said pair of grid-like electrode plus a constant.

4. The method of effecting distortionless demodulation of signal modulated carrier frequency oscillations by means of an electron discharge tube having a control grid electrode, a cathode electrode and a plurality of auxiliary electrodes which includes the steps of, maintaining said control grid negative relative to said cathode, impressing signal modulated carrier frequency oscillations substantially in phase on a pair of said auxiliary electrodes, and maintaining' the same at different direct current potentials with respect to said cathode, one of said auxiliary electrodes being maintained at twice the potential of the other auxiliary electrode plus a constant.

5. The method of effecting distortionless demodulation of signal modulated carrier frequency oscillations by means of an electron discharge tube having a control grid electrode, a cathode electrode and a plurality of auxiliary electrodes which includes the steps of, impressing the signal modulated carrier frequency oscillations on said control grid electrode, applying heterodyning potentials substantially in phase to a pair of said auxiliary electrodes, and maintaining the same at different direct current potentials with respect to said cathode, one of said auxiliary electrodes being maintained at twice the potential of the other auxiliary electrode plus a constant.

6. In a signal demodulating system an electron discharge tube having an input electrode coupled with a source of signal modulated carrier frequency oscillations and an output electrode coupled with an output circuit, said tube having a cathode, a plurality of auxiliary electrodes interposed between said input and output electrodes, a source of heterodyning potentials, separate circuits coupling said source of heterodyning potentials to said auxiliary electrodes for applying heterodyning potentials of different amplitude thereto, and means for maintaining said auxiliary electrodes at different direct current potentials with respect to said cathode.

7. A signal demodulating system comprising an. electron discharge tube having a grid electrode, a cathode, an anode coupled with an output circuit, and a plurality of auxiliary electrodes interposed between said grid and anode electrodes, a circuit including a source of potential connecting said gridto said cathode for maintaining said grid negative relative to said cathode, a source .of signal modulated oscillations, separate circuits for coupling said source of signal modulated oscillations in phase to said auxiliary electrodes to apply thereto oscillations of diiferent amplitude, and means for maintaining all of said auxiliary electrodes at different direct current potentials with respect to said cathode, one of said auxiliary electrodes being maintained at twice the potential of another auxiliary electrode plus a constant. I

8. A signaling device comprising, an electron discharge tube having an anode, a cathode, and a control grid electrode, a circuit for applying signal modulated carrier frequency oscillations to said control grid and cathode, a load circuit connected between said anode and said cathode,

, an means for effecting linear demodulation of the signal modulated carrier frequency oscillations impressed on said control grid and cathode comprising, a plurality of auxiliary grid-like electrodes interposed between said control grid and anode, circuits for maintaining said auxiliary electrodes at different direct current potentials with respect to said cathode, a source of oscillations, and couplings between said source of oscillations and said last named circuits for impressing oscillations therein in substantially like phase relation.

9. A signaling device comprising, an electron discharge tube having an anode, a cathode, and a control grid electrode, and a plurality of auxiliary electrodes, a circuit for applying oscillations between said control grid and cathode, a load circuit connected between said anode and said cathode, a circuit for applying signal modulated carrier frequency oscillations in phase to a plurality of the auxiliary electrodes and means for maintaining said auxiliary electrodes at diiferent direct current potentials with respect to said cathode.

10. The method of efiecting substantiallydistortionless demodulation of signal .rnodulated carrier frequency oscillations by means of an electron discharge tube having a control electrode, a cathode electrode, and a plurality of auxiliary electrodes which includes the steps of, impressing signal modulated carrier frequency oscillations on said control electrode, applying other oscillations of like frequency and of different amplitude substantially in phase to a pair of said auxiliary electrodes, and maintaining the said pair of auxiliary electrodes at different direct current potentials with respect to said cathode.

11. A method of signaling by means of an electron discharge tube having a cathode electrode, a control electrode and a plurality of auxiliary electrodes which includes the steps of,

impressing oscillations on said control electrodes, impressing signal modulated oscillations of difierent amplitude substantially in phase on a pair of said auxiliary electrodes, and maintaining the said pair of auxiliary electrodes at different direct current potentials with respect to said cathode, one electrode of said pair of auxiliary electrodes being maintained at a potential equal to the potential of the other electrode of said pair of auxiliary electrodes plus a constant.

12. The method of effecting substantially distortionless demodulation or signal modulated carrier frequency oscillations by means of an electron discharge tube having a control grid and cathode electrode and a plurality of auxiliary electrodes which includes the steps of, maintaining said control grid negative relative to said cathode, impressing signal modulated carrier frequency oscillations of diiferent amplitude substantially in phase on a pair of said auxiliary electrodes, and maintaining the pair of auxiliary electrodes at different direct current potentials with respect to said cathode.

13. The method of effecting substantially distortionlessdemodulation of signal modulated carrier frequency oscillations by beating the same with unmodulated oscillations in an electron discharge tube having a cathode, a control grid and a plurality of auxiliary electrodes located at different distances from said cathode which ineludes the steps of, applying one of said oscillations to said control grid, applying the other of said oscillations substantially in phase to a pair of said auxiliary electrodes, the amplitude of the oscillations applied to the auxiliary electrode of said pair nearest um noi-hOde being materially greater than the amplitude of the oscillations applied to the other electrode of said pair of auxiliary electrodes, and maintaining said auxiliary electrodes at different direct current potentials relative to said cathode.

14. The method of efiecting substantially dis- .tortionless demodulation of signal modulated carrier frequency oscillations by means of an electron discharge tube having a control grid electrode, a cathode, and two auxiliary electrodes spaced at difierent distances from said cathode which includes the steps of, impressing signal modulated carrier frequency oscillations on said control grid, applying other oscillations of different amplitude substantially in phase to said pair of auxiliary electrodes, the other oscillations applied to said auxiliary electrode nearest said cathode being of substantially greater amplitude than the other oscillations supplied to the other of said auxiliary electrodes, and maintaining the auxiliary electrodes at different direct current potentials with respect to said cathode.

15. The method of signaling by means of an electron discharge tube having a cathode electrode, a control grid and a plurality of grid-like electrodes spaced at different distances from said cathode which includes the steps of, impressing oscillations on said control grid, impressing signal modulated oscillations of different amplitude substantially in phase on a pair of said grid-like electrodes, the amplitude of the oscillations of different amplitude impressed on the grid-like electrode of said pair nearest the cathode being materially greater than the amplitude of the modulated oscillations impressed on the other electrode of said pair, and maintaining the gridlike electrodes at different direct current potentials relative to the cathode.

16. The method of effecting substantially distortionless demodulation of signal modulated carrier frequency oscillations by means of an electron discharge tube having a control grid, a cathode electrode, and a plurality of auxiliary electrodes located at different distances from the cathode whichincludes the steps of, maintaining said control grid negative relative to said cathode, impressing signal modulated carries frequency 10 oscillations on one of said auxiliary electrodes,

impressing signal modulated carrier frequency oscillations of like phase and of materially greater amplitude on another of said auxiliary electrodes located nearer to said cathode than said one auxiliary electrode, and maintaining the auxiliary electrodes at different direct current potentials relative to the cathode.

HAROLD OLAF PETERSON. MARTIN KATZIN. 

